Figure 5. Proposed reaction scheme for [4Fe-4S] RirA cluster conversion/degradation.

Reaction scheme used to fit time-resolved ESI-MS data for [4Fe-4S] RirA. Values of rate constants (k) are given in Table 1. Note that the ESI-MS data cannot distinguish between [3Fe-4S]¹⁺ and [3Fe-4S]⁰ forms (boxed), so these are treated only as [3Fe-4S] in the kinetic model. They can be distinguished by EPR, however (see below), and the kinetic EPR data support the proposal that [3Fe-4S]⁰ is susceptible to oxidation to [3Fe-4S]¹⁺ in the presence of O₂. Reactions that are enhanced in rate (i.e. have increased associated rate constants) in the presence of O₂ are indicated by red arrows. The initial [4Fe-4S] cluster is coordinated by three Cys residues and one unknown ligand, illustrated in the figure as ‘X’. The structure of the [3Fe-3S] species proposed here is based on the recently reported small molecule [3Fe-3S] species in which iron and sulphur form a hexagonal ring (Lee et al., 2016). The faded portion of the figure (involving steps marked k₅, k₆ and k₇) represents a minor pathway that most likely arises from ionisation-induced damage.